When blood
flows in capillary tubes of less than 300 μm diameter its
viscosity is less than might be expected and the percentage of red blood cells
(haematocrit) is also reduced. These observations are well known for Pouiseille flow – i.e. flow which is driven by pressure
variation along a tube. Such mechanisms
in Couette flow – i.e. flow in a small gap between a
moving boundary and a stationary boundary – are just about completely
unknown. An understanding of red blood
cell dynamics in these circumstances is important as this affects the design of
rotary blood pump. If a red blood cells are exposed to high shear stresses its
membrane will rupture and the cell contents are lost (a process known as haemolysis). There
is evidence (as yet scientifically untested) that red cells might be excluded
from such small gaps in rotary blood pumps meaning that these cells cannot be
lysed, but also that the viscosity of the blood in the
bearings will be lower than might be expected and the bearing forces developed
by blood hydrodynamic bearings will be weaker than
expected.
In this
research we aim to measure the extent of red cell exclusion as a function of gap
and speed experimentally, to develop a description of the mechanical forces and
processes at work and then to develop guidelines for the correct configuration
of Couette gaps in rotary blood pumps. The research approaches used include
laser-based diagnostics of experimental blood flow, computational models both of
particlulate flow and of continuum fluid flow and
analytical models of particulate flow.
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